Thanks Tim, I then don't know what am I doing wrong; I have many questions in a Cat. I made copies of the questions at the same Cat. Then created a new Cat. where I move the copies to. When I go to these copies question by question and delete the dataset at the last step thus keeping only one possible Answer, I notice that original questions are also restricted to one single answer!
What I am trying to do is to get a snap of a calculated multi-choice question with only one possible answer without affecting the original question.